Programa nacional de sustitución integral de sustitución de cultivos de uso

As mentioned before, by incorporating active carriers into membranes, permeation of certain gases can be enhanced by using facilitated transport membranes. To some extent, this type of membranes is similar to biological cell membranes. The carrier in a facilitated transport membrane interacts or reacts specifically and reversibly with a target permeant to form a permeant-carrier complex, which diffuses to the downstream side of the membrane, where the complex decomposes and releases the permeant and carrier.

There are two major types of facilitated transport membranes based on the mobility of the carriers in membranes. The carrier molecules can be mobile in the membrane (e.g., liquid membranes) where the carriers can diffuse in the membrane, or immobilized in the membrane matrix where the carriers are fixed to certain sites of the matrix. Both types of facilitated transport are discussed below.

2.3.4.1 Liquid membranes

Since diffusivities in liquids are orders of magnitude higher than diffusivities in solids, much higher permeabilities in liquid membranes can be expected than permeabilities of typical solid membranes. Thus, liquid membranes have been studied in past years, but their practical applicability is still limited due to membrane instability.

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Generally, liquid membranes are comprised of supports, liquids and carriers. The selection of supports is very important to the separation performance of liquid membranes. Firstly, the support could affect the overall permeability if its porosity is not large enough. In addition, a suitable support should provide a sufficient stability of the liquid membrane. The liquid membranes can be further subdivided into immobilized liquid membranes (ILM) and supported liquid membranes (SLM). A liquid is held inside the pores of a porous support by means of capillary forces in an ILM, where the support has to be wettable by the liquid under such condition. In a SLM, the liquid is located on top of the porous support [Krull et al., 2008].

Because of similar magnitude of gas diffusivities in a liquid, the selectivity of a liquid membrane is based mainly on the sorption selectivity of a gas pair in the liquid. In case the sorption selectivity is very low or lacking, carrier species interacting selectively with a gas species may be employed. The carriers should be soluble in the liquids, and a permeant molecule

(e.g., CO2) should be reversibly bound to a carrier and transported across the membrane via

facilitated transport mechanism, resulting in an enhanced permeability. Materials with amine

functional groups are generally considered as possible carriers in liquid membranes for CO2

permeation. Thus, the amines used for CO2 absorption may be the potential carriers for CO2

separation by facilitated transport based on the fast reaction kinetics of amine and CO2.

Monoethanolamine, diethanolamine and ethylenediamine have been utilized as the facilitating

carriers for CO2 separation in either SLM or ILM configuration [Teramoto et al., 1996, 1997;

Gorji and Kaghazchi, 2008]. Besides, dendrimers containing abundant terminal groups in a

highly branched molecular structure are another potential carrier materials for CO2 transport

[Inoue, 2000]. Poly(amidoamine) (PAMAM) dendrimers of generation 0 with an ethylenediamine core have a large amount of primary and tertiary amine groups, and they are

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shown to be effective carriers for CO2 separation [Kovvali et al., 2000; Kovvali and Sirkar, 2001,

Duan et al., 2006; Kouketsu et al., 2007].

The long term stability of a liquid membrane is largely dependent on the volatility of the membrane liquid. Hence, low to non-volatile liquids (e.g., glycerol, ionic liquids, and liquid molten salts) are desired to avoid a breakdown of the membrane function due to evaporation. However, the use of aforementioned liquids is still limited even though they can improve the membrane stability to some extent because the carrier washout under a higher transmembrane pressure cannot be prevented [Krull et al., 2008]. Although the permeability and selectivity of liquid membranes seem to have improved significantly over the past decade, their long-term stability remains a major technical challenge for practical applications.

2.3.4.2 Fixed-site-carrier membranes

One way to improve membrane stability is to immobilize the carriers within the membranes. In a fixed-site-carrier membrane, the carriers are covalently bond directly to the polymer backbone. Carrier molecules can only vibrate within some distance near the equilibrium position but cannot move freely. The target permeant reacts at one carrier site and then hops to the next carrier site, moving from upstream to downstream side via the “hopping” mechanism [Cussler et al., 1989]. Obviously, the interaction between permeating species and carriers is depressed in fixed-site-carrier membrane as compared to liquid membranes with mobile carriers. This results in a relatively lower permselectivity in fixed-site-carrier membranes. Nevertheless, from a membrane stability point of view, fixed-site-carrier membranes are preferred.

Currently, the research of fixed-site-carrier membrane for CO2 separation has mainly

focused on amine based polymers. Various amine based polymers are used: polyvinylamine with primary amine groups [Kim et al., 2004, 2013; Deng et al., 2009; Deng and Hagg, 2010; Yuan et

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al., 2011; Qiao et al., 2013], polyallylamine with primary amine groups [Zou and Ho, 2006; Huang et al., 2008; Cai et al., 2008], pentaerythrityl tetraethylenediamine dendrimer with both primary and secondary amine groups [Wang et al., 2007], and poly(N-Vinyl-γ-sodium aminobutyrate) containing secondary amine and carboxylate groups [Zhang et al., 2002a]. Fixed- site-carrier membranes based on these polymers all exhibited excellent separation performance

in terms of CO2 permeation rate and selectivity. For example, polyallylamine/poly(vinyl alcohol)

blend membranes showed a selectivity for CO2 over N2 and CO2 over CH4 of 80 and 58,

respectively [Cai et al., 2008].

In fixed-site-carrier membranes containing sufficient amine groups, CO2 is mediated based

on the weak acid-base reaction between CO2 and amine moiety in dry state. While in water-wet

membrane, water also plays a role in the CO2 transport because of membrane swelling [Liu et

al., 2008]. The reaction of CO2 with primary and secondary amines is usually described by the

zwitterion mechanism including the following steps [Little et al., 1992; Zou and Ho, 2006; Yuan et al., 2011]: (2-3) (2-4) 𝑅₁𝑅₂𝑁𝐻+_𝐶𝑂𝑂−_{+ 𝐻} 2𝑂 ⇌ 𝑅1𝑅2𝑁𝐶𝑂𝑂−+ 𝐻3𝑂+ (2-5) 𝑅1𝑅2𝑁𝐶𝑂𝑂−+ 𝐻2𝑂 ⇌ 𝑅1𝑅2𝑁𝐻 + 𝐻𝐶𝑂3− (2-6)

where 𝑅1 and 𝑅2 are functional groups or hydrogen. In the first step, CO2 reacts with primary or

secondary amines to form zwitterion as an intermediate. The next step is the subsequent removal

of the proton from the zwitterions by a base or H2O, forming carbamate ions. The carbamate ion

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conveyed in the CO2-amine complex forms in terms of carbamates and bicarbonates. On the

other hand, tertiary amines cannot react with CO2 directly. It is suggested that such amines have

a base-catalytic effect on the hydration of CO2 [Donaldson and Nguyen, 1980; Vaidya and

Kenig, 2007; Yu et al., 2010], represented as follows:

(2-7)

CO2 reacts with tertiary amines and water to produce bicarbonate within the membrane, diffusing

in the form of bicarbonate ions.

As mentioned earlier, owing to the enhanced transport of CO2 bound to the carriers, SLMs

or ILMs display both high CO2 permeance and CO2/light gas selectivity. However, the possible

instability issue of the membranes caused by the solvent evaporation and carrier washout poses a great challenge if large scale industrial application is considered. In order to overcome this limitation, research efforts were devoted to develop facilitated transport membranes with fixed- site-carriers. By covalently bonding the carriers to the polymer backbone, fixed-site-carrier membranes normally show improved stability than the liquid membranes. In view of the above considerations, facilitated transport membranes containing both mobile and fixed carriers may

combine the excellent CO2 permselectivity and good stability since the additional mobile carriers

could be well retained in the polymer structure and both the mobile and fixed carriers can be

used to facilitate the transport of CO2 molecules. Pioneering work of the novel facilitated

transport membranes containing poly(allylamine) or polyethylenimine as the fixed carriers and

amino acid salts as mobile carriers designed for CO2/light gas separation have been done recently

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